We have discovered that unilateral removal of the massive cortical input to the dentate gyrus triggers a rapid "synaptic shuffle." All remaining extrinsic and intrinsic synaptic fields in the dentate gyrus are reorganized and the fiber tract from the homologous cortex becomes functional. The GABAnergic interneurons, which reside within the denervated zone, increase the activity of glutamic acid decarboxylase so that they form new terminals or change the enzyme activity in existing ones. All changes observed are compatible with denervation inducing axon collateral sprouting, but other possibilities cannot be rigorously excluded as yet, in most cases.